Without limiting the scope of the invention, its background is described in connection with novel apparatus and methods for non-destructive pipeline inspection. Pipelines require periodic testing to insure the absence of defects that may ultimately result in loss of product or catastrophic pipeline failure. Pipeline defects may include loss of coatings, corrosion, stress corrosion cracking (SCC), and circumferential and axial flaws. Non-destructive testing (NDT) methods include internal inspection using smart pigs that are adapted to run the length of the pipeline while conducting testing. Testing methods include physical surface analysis, corrosion testing and crack detection. Corrosion manifest as metal loss, as well as crack detection, are most typically detected by magnetic flux leakage (MFL) although ultrasound testing has also been applied to corrosion and crack detection.
Electromagnetic acoustic transducer (EMAT) technology is relative newcomer to the field of non-destructive pipeline integrity testing. EMAT technology involves the generation of ultrasonic acoustic waves in electrically and magnetically conductive materials by the combined interaction of magnetic fields together with a relatively high frequency (RF) field generated by electrical coils. The low frequency or static field generated by the magnets interacts with eddy currents in the RF field to generate ultrasonic acoustic waves caused by Lorentz forces or magnetostriction in the test material. Depending on the configuration of the electrical coil and the magnetic field, a number of different types of ultrasonic acoustic waves can be generated in the tested material including, among others, shear waves. Generated shear waves can be shear vertical (SV) or shear horizontal (SH) waves depending on the method of excitation. Excitation of SH waves may result in concomitant excitation of SV waves, which propagate in the same direction as the SH waves but at smaller amplitudes. An EMAT designed to utilize SH waves employs a row of magnets with alternating polarities that produces a shear wave whose polarization is in the plane of the surface to be tested. Defects such as corrosion or cracks in the pipeline walls create detectable reflections in the generated shear waves.
The EMAT in-line inspection method described by Paige et al. (PCT/GB02/04031, “Pipeline Inspection Pigs”, published as WO03/021249 and US2005/0072237) uses rings of transmit/receive transducers wherein each ring includes two to three sets of pairs of transmitting and receiving transducers (T1, R1, T2, R2, etc), with the pairs being co-planar and evenly circumferentially spaced close to the internal surface of the pipe. All transmitting EMAT in this set are switched on simultaneously. To avoid measurement interference by ultrasound waves circulating around the circumference of the pipe, a time interval between emission of a given ultrasound wave and emission of a consequent ultrasound wave is established to be long enough for all previous ultrasound waves to have decayed by the time of emission of the next ultrasound wave. To assure unambiguity of the location of defects and to increase probability of their detection, the rings of transmit/receive transducers described in the above mentioned PCT/GB02/04031 are spaced along the axis at a distance that is long enough to exclude ultrasonic waves of one ring from being received by transducers of the adjacent ring. The transmit/receive transducers of a given ring are installed with an angular shift relative to the transmit/receive transducers of the adjacent ring. While this method has an advantage of using a small number of EMAT to ensure low power consumption and therefore maximum effective range of the in-line inspection tool, in this method the several transducers sets are shifted along the axis, resulting in an increased length of the in-line inspection tool. Moreover, the long time interval between emissions of ultrasound waves as the tool is moving through the pipeline establishes a relatively long distance along the pipeline axis between where ultrasound waves are generated and where crack-like defects can be detected thus limiting the in-line inspection length resolution along the axis of the pipe.
A further EMAT technology in-line inspection method, described by Alers et al. (CA2592094, “Device for Testing Ferromagnetic Component Walls Without Destruction of the Same”, published as WO06/069684), involves transmitting EMATs generating ultrasound waves that propagate at a 10-60 degree, and preferably a 20-50 degree, angle to the direction of magnetic field. For each direction of ultrasound wave emission from each transmitting EMAT there is one corresponding receiving EMAT located some distance away from the zone of the ultrasound wave propagation according to the emission direction and which receives only ultrasound waves that are reflected from crack-like defects in the testing area. A further separate reference EMAT is employed that is located outside the testing area and which receives ultrasound waves that have traveled through the testing area. An advantage of this method of inspection is that the generated ultrasound waves, while propagating along the pipe wall, travel away from the pipe circumference where they have been generated. This arrangement avoids ultrasound wave circulation and influence on subsequent measurement of following ultrasound waves. The main disadvantage of this method is that in order to cover the whole circumference of the pipe there is a need for at least two rows of EMATs where one row forms transmitting EMATs covering the whole circumference of the pipe and the other forms receiving EMATs. The initial ultrasound wave is received with one EMAT, and the ultrasound wave reflected from a crack is received with a second EMAT. Because ultrasound waves travel away from the pipe circumference where they have been generated, a large number of EMATs are used to cover the whole circumference of the pipe. However, EMATs are the most power-consuming elements in the electronic system of such an inspection tool. Because batteries installed in a tool have limited capacity, the number of EMATs is a factor which limits hours of service of the in-line inspection tool, and therefore its maximum effective range. Having a large number of power-consuming EMATs is a factor that limits the maximum operating range of the tool.
What is needed is an EMAT inspection method and apparatus that effectively detects pipeline wall defects without signal interference but features a compact design and conservative power consumption.